In the manufacture of metal sheets including steel sheets, properties are adjusted in such a manner that phase transformation is induced by cooling the metal sheets in a continuous annealing line after heating. In recent years, high-tensile strength steel sheets have been increasingly used in the automotive industry for the purpose of achieving the weight reduction and crash safety of automobiles. In order to respond to such a demand trend, the importance of rapid cooling techniques advantageous in manufacturing high-tensile strength steel sheets is growing. A water quenching method having the highest cooling rate is generally a way to rapidly cool a steel sheet in such a manner that cooling water is applied to the steel sheet from quenching nozzles placed in water at the same time that the heated steel sheet is immersed in water. In this method, there is a problem in that shape defects are caused in a metal sheet by out-of-plane deformation including camber and wavy deformation.
Patent Literature 1 proposes a technique in which bridle rolls are provided upstream and downstream of a rapid quenching section as tension-changing means capable of changing the tension of a steel sheet subjected to a rapid quenching step for the purpose of reducing the wavy deformation of a metal sheet that occurs during rapid quenching in a continuous annealing furnace.
Patent Literature 2 proposes a technique in which, in consideration of the fact that shape defects are caused because compressive thermal stress is generated in lateral directions of a metal sheet at a quenching start temperature (cooling start temperature) and therefore the metal sheet buckles, out-of-plane deformation is reduced in such a manner that both sides of the metal sheet are pinched at a region having the compressive stress generated in the lateral directions of the metal sheet by cooling or another region close thereto.